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UNIT 5 NOTES
Chapter 13 Sections 1-4
In the last unit you learned how cells communicate with each other and about
the cell cycle. When cells divide at the end of that cycle they go through mitosis
and cytokinesis. For simple, single celled organisms like an amoeba, that is also
reproduction and it is asexual. Some simple multicellular organisms also
reproduce asexually, but for large multicellular organisms, that is just not going
to work. There has to be sex. And what you are thinking is not what I mean.
These notes are the PG version, remember? Sex is the transfer of genes from
one cell to another. Back in chapter 11, I told you about bacteria that would
communicate sometimes that they wanted to have sex. There are bacteria out
there that can take in genetic material either from another cell or sometimes
stray pieces in the environment. Sometimes they will reproduce after, and
sometimes not. It’s a crazy world out there.
In this unit you will learn about sexual reproduction and genetics. There is A LOT
of vocabulary and math. You will not be tested directly on vocabulary, but if you
don’t know what the words mean, test questions will be hard to understand, so
learn the vocab. You will also be reading an extra book during your Christmas
vacation that pertains to this subject, and I hope you enjoy it. Be prepared to
discuss it, which means everybody talks, not just me.
Here we go. Genetics is the scientific study of heredity and variation. Heredity
is the transmission of traits from one generation to the next. Variation is
demonstrated by the differences in appearance that offspring show from parents
and siblings. Differences such as eye color, like blue or brown. Eye color is an
inherited trait and all siblings will inherit an eye color gene from each parent, but
the versions of that gene each sibling gets may be different. These different
versions of a gene are called alleles. That is variety and it is good.
What are the units of heredity?
How are they passed to the next generation?
Each one has a specific location on a chromosome called __________________.
If you read the definitions for asexual reproduction and clone, you should
realize that asexual reproduction is, in effect, producing clones. All the offspring
are identical to each other and the single parent. How is sexual reproduction
different?
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Here is another cycle for you. The life cycle. You are born, you grow,
reproduce, and then the cycle continues with your offspring. Most organisms you
are familiar with follow basically the same life cycle. We will stick with using
humans as an example so as to follow the book, and since most of you are
human that makes the most sense anyway.
Humans have two categories of cells: gametes and somatic cells.
Gametes are ____________________ and ____________________.
Somatic cells are all the rest of them. Each of these cells has how many
chromosomes?
So that is how many pairs?
Scientists can make an ordered display of all the chromosome pairs in a cell
called a karyotype. The pairs are arranged from largest (pair #1) to smallest.
This is actually done using a picture of the cells chromosomes that have been
enlarged and stained. The two chromosomes in each pair are called
homologous chromosomes or homologs. The prefix homo- means the same,
so these two chromosomes make a pair because they carry the same genes in
the same order. You received one from each pair from your mother and the
other from your father. Notice how the homologous chromosomes colors match
in Fig. 13.3 pg. 250 , slide 8. Also notice the pairs are numbered up through 22.
Your answer to the question above should have been 23. So where is that pair?
Look just above pair 22 and you will see one mismatched pair labeled X and Y.
What are X and Y?
Is this person male or female?
What does the other sex have?
What are the other 22 pairs called?
Why is the Y chromosome so much smaller than the X? Are men missing
something? Do they shop? No. Can they multi-task? No. Are they organized? Not
usually. And do they talk about their feelings? Never. Guess that explains that.
This is just my theory, and it has not yet been scientifically proven, but I bet it
would win hands down in an opinion poll.
Since your chromosomes come in pairs, one from mom and one from dad, you
have two complete sets IN EVERY SOMATIC CELL.
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Cells with two complete sets are referred to as _____________________ or 2n.
A gamete has only one complete set and is called ____________________ or n.
Each gamete will have only one sex chromosome. What will be in an egg? _____
How is that different in a sperm cell?
So who determines the sex of a child, mom or dad?
What is fertilization?
What is the fertilized egg called?
What process does it use to grow into an adult?
What process is used to make gametes?
These are the ONLY cells produced this way.
Slide 14, Fig. 13.5, pg. 251, shows you the human life cycle. Notice how it
alternates between haploid and diploid stages. You are a diploid organism that
forms haploid gametes by meiosis. During fertilization one of your gametes will
fuse with the gamete of another and produce a new diploid organism that will
cost you hundreds of thousands of dollars over the course of your lifetime.
Diploid  haploid  diploid.
In mammals and many other organisms, the life cycle consists of a diploid
organism that creates haploid gametes which unite to create a new diploid
organism. Gametes do not develop on their own. (Usually, there are exceptions.)
This alteration of meiosis and fertilization occurs in all sexually reproducing
organisms. Plants, algae, and fungi undergo an alternation of generations where
the haploid cells do grow into some sort of functional organism. These haploid
structures produce gametes that join in fertilization and produce the diploid
generation which then produces haploid spores or organisms. These life cycles
are explained a bit more in slides 18 through 24 and in the book in detail. It
takes some time to understand each of these fully, and it is NOT required of you
for this course. You do need to understand the similarities that each life cycle
alternates between haploid and diploid even though different types of organisms
spend different amounts of time in each stage. Use slide 18, Fig. 13.6 pg. 252, to
compare. Although the life cycles are different these organisms all share a sexual
life cycle that produces variation through meiosis, crossing over, and fertilization.
Let’s take a look at meiosis and see how it magically reduces the number of
chromosome sets from two to one.
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Meiosis is a type of cell division that reduces the chromosome number by half.
Therefore, it is also called a reduction division. There are similarities with mitosis,
but also many differences. You need to concentrate on those and not concern
yourself in memorizing the names of the phases. Learn what happens overall and
why.
The first thing that happens in any type of cell division is replication. Cell
division by its nature makes more cells, so more chromosomes must be made to
give to those cells. Even here in meiosis where cells are getting fewer
chromosomes there still needs to be replication. One difference between mitosis
and meiosis is that there are two divisions in meiosis, meiosis I and meiosis II.
This means that there will be twice as many cells when meiosis is done, four
cells instead of two. Each of these four cells will only have half the number of
chromosomes as the parent cell. Not just any half, though. This is a very
organized division so each daughter cell will get one complete set, haploid cells
instead of the diploid cells formed by mitosis.
Here is how it works: you must start with a diploid cell (2n). Then all the
chromosomes replicate (still 2n, sister chromatids are considered one
chromosome, but there is twice as much DNA). Now comes the BIG DIFFERENCE
that makes meiosis different from mitosis. Each chromosome lines up in the
middle, not by itself, but paired up with its homolog. This creates something called
a tetrad, four sister chromatids in a row. When the cell divides in meiosis I, these
pairs are separated. Since each cell gets only one chromosome (replicated, two
sister chromatids) these are haploid (n) cells. This first division has reduced the
chromosome number. Each of these cells then undergoes another division that
separates the chromatids, resulting in four haploid daughter cells.
Fig. 13-7-3
Interphase
Homologous pair of chromosomes
in diploid parent cell
Chromosomes
replicate
Homologous pair of replicated chromosomes
Sister
chromatids
Diploid cell with
replicated
chromosomes
Meiosis I
1 Homologous
chromosomes
separate
Haploid cells with
replicated chromosomes
Meiosis II
2 Sister chromatids
separate
Haploid cells with unreplicated chromosomes
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Fig. 13.7 pg. 253, slide 27 is pictured above. This is what you need to
understand of meiosis. Next to this figure you should write n or 2n for each level
so you can see exactly where the reduction occurs. Notice too, that some of the
cells got the red chromosome (on the right) and some got blue (left). Genetically
they are different. Remember too, that there are 22 more chromosomes in a real
cell all lining up and separating. How all these chromosomes line up depends on
which chromosomes, maternal or paternal origin, end up in which cells. This
produces much of the variation you get from sexual reproduction. We will do a
‘circle activity’ on this in class so you see what I mean.
***To summarize, let’s compare mitosis and meiosis:
MITOSIS
produces 2 genetically identical
diploid cells
cells divide once
2n  2n
used for growth, repair and
asexual reproduction
MEIOSIS
produces 4 genetically different
haploid cells
cells divide twice
2n  n
used only to make gametes
Don’t forget that both processes follow the same basic steps: replication,
alignment, and separation. The big difference is in the alignment which leads to
the difference in outcome. Use slide 30, Fig. 13.9 pg. 256 to study.
How is it we have different alleles to make different combinations in the first
place? Why doesn’t everyone just have brown eyes? Well, ‘cause sometimes
there are changes made in the DNA of a gene. We call these changes
mutations. DNA does not always replicate correctly. Sometimes mistakes are
made. There are repair processes in the cell to fix mistakes, but they don’t
always work. (These processes will be discussed in Unit 6.) A mistake that
doesn’t get fixed, results in a change in the DNA that gets passed on to the next
generation. These mutations are the original source of genetic diversity creating
different versions of genes known as alleles. Organisms that reproduce
asexually can also have mutations and that is their only source of variation.
There are other ways to produce variations during meiosis too. One of these is
independent assortment. This is one of Mendel’s laws of genetics that we will
discuss in the next chapter, but here is how it is explained physically. This is a
major source of genetic variation in sexually reproducing organisms. It goes back
to what I said above: it all depends on how the chromosomes line up in meiosis I.
Each pair of chromosomes can line up in one of two ways. The maternal
chromosome can be on the left and paternal on the right, or vice versa. The more
chromosomes an organism has, the more different combinations you can make,
and the more variety you get. Independent assortment means that each
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chromosome pair lines up which ever way it wants and it has NOTHING to do with
any of the other pairs. Each pair does its own thing. You will see this in class in the
circle paper activity and it is shown in slide 36, Fig. 13.11 pg. 258.
Another source of genetic variation that happens only during meiosis is crossing
over. Crossing over produces new combinations of alleles, mixing maternal and
paternal characteristics onto the same chromosome. These new chromosomes
that have a mix of maternal and paternal genes are called recombinant
chromosomes, the DNA has been recombined. This process starts when the
homologous chromosomes pair up. They twist around each other (called
synapsis) and complimentary pieces of DNA are exchanged between nonsister
chromatids. Sister chromatids are identical so exchange between them would not
result in any changes to the chromosomes. Crossing over results in new
combinations of alleles on the same chromosome. This makes it possible for you
to inherit, on one chromosome, a gene from your mother’s mother and one from
her father. Your book has a good diagram of this in Fig, 13.12 pg. 259, slide 39.
Fertilization is necessary to restore the diploid chromosome number. Since
millions of sperm are produced at one time, which one gets to the egg first is
really just dumb luck. It is a totally random event, and would be different every
time if there could be do-overs. So, if a woman were to go to a sperm bank that
specializes in super geniuses, she still has no guarantee that’s what she’ll geteach sperm is different. Maybe the not-quite-as-smart sperm swim just a bit
faster. Who knows?
CHAPTER 14 SECTIONS 1-4
Gregor Mendel is a name you should remember from Living Environment. He’s
the monk that that did all those experiments with pea plants and figured out the
basic principles of heredity- without knowing anything about genes or
chromosomes. He also didn’t have any computers or calculators, just pencil and
paper to do all his math. Think about that next time you do your math
homework, and appreciate that graphing calculator that also let’s you play Mario.
Mendel was lucky in several ways. The pea plants he chose to work with had
complete flowers, both male and female parts. The male parts could be easily
removed to prevent self fertilization if necessary, thereby controlling the mating
of plants. They also came in several varieties with different characters. A
character is a feature of an organism such as flower color. Different character
variants, like different colors, are traits. By working with characters that only
had two traits, he was able to formulate his laws.
He started his experiments with plants that were true-breeding, plants that
produce offspring of the same variety when they self-pollinate, like always
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having purple flowers. A genetic experiment is usually called a cross, especially
in plants, and can be represented by an X. Standard procedure for genetics
problems is that the parent generation is P, the offspring of the two parents is
the F1 generation, and crossing (mating) two F1 individuals gives you the F2
generation.
Mendel started out with hybridization experiments, crosses between two truebreeding plants with contrasting traits, like purple X white flowers. The F1
hybrids from this experiment were all purple. Where did the white go? He
wondered, so he crossed the F1 hybrids (let them self pollinate) and found in the
F2 generation that most of them were purple, but some were white. In fact, one
white for every three purples, a 3:1 ratio. This was great news! The white trait
had not disappeared, but was only hidden by the purple in the hybrid. He found
the same results when he repeated his experiment with different characters.
From these experiments we get more genetics terminology: the trait that shows
in a hybrid is dominant and the one that does not is recessive. Mendel also
reasoned that these traits must be passed from one generation to the next by
some sort of heritable factor. At the time he did not know what that could be,
but now we know and we call them genes.
Mendel, like any good scientist, now had to explain his results. In his
explanation, or model, he came up with four concepts. Some of these concepts
you are already familiar with from chapter 13, so keep meiosis in mind as you
work on this chapter. The two are related. His first concept is that alternative
versions of genes account for the variations like the purple and white.
What are different versions of genes called?
Each gene has a specific place on a chromosome called a what?
Mendel’s second concept almost seems obvious now. For each character you
have, you inherit two alleles, one from each parent. These alleles can be the
same (true-breeding) or different (hybrid). This brings us to his third concept.
If those two alleles are different, then the one that determines appearance, the
one you see, is the dominant allele, and the other is the recessive one.
Mendel’s fourth concept is now known as the Law of Segregation so it’s pretty
important. This law states that the two alleles an organism has will segregate, or
separate, during gamete formation (meiosis) and go to different gametes. Go
back to page 4 and look at the meiosis diagram again. Each gamete gets only
one of a pair of homologous chromosomes. It wasn’t until a hundred years or
more after Mendel’s work that somebody put two and two together and finally
realized that what Mendel had done was explain what they were seeing in their
microscopes as they watched cells go through meiosis.
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Now, let’s sum this up. Each gene has variations called alleles. You have two for
each character. You got one from each parent. If they are not the same, the
dominant one will determine what you look like. When you make gametes each
allele you have will separate from the other and go into a different gamete. A
way to show this pictorially, and show how Mendel got his 3:1 ratios, is with a
Punnett square. You learned how to do these in eighth grade. We will do a
refresher in class since it has been quite a while. These are helpful tools to use
to understand these concepts. As you look at and use these squares, notice the
notation for the alleles: a capital letter for a dominant allele, such as R for red
and the recessive allele is given the same letter, but in lower case, such as r for
white.
The following terms are necessary for you to know to be able to understand any
text, conversations, or classwork involving genetics. Use your book or the
PowerPoint for definitions and learn them now. Examples would also help you.
Homozygous –
Heterozygous –
Phenotype –
Genotype –
If an organism shows the dominant phenotype it could be homozygous or
heterozygous. How will you determine the genotype?
Explain how the results determine the parental genotype.
Mendel came up with another law when he decided to do some more difficult
experiments. Up til now he was just looking at one character at a time. A hybrid
in the F1 generation for just one character is called a monohybrid, monomeaning one. Mendel then decided to get crazy and track two characters at
once. Starting his cross with individuals that were true-breeding for two
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characters, resulted in individuals in the F1 generation that were hybrid for two
characters or dihybrids. Look at slide 64, Fig. 14.8 pg. 268. It shows two
possible outcomes. The smaller square shows the result one would get if the two
characters were always inherited together as a unit. The larger square shows the
results if each character was inherited independently. When Mendel performed
the crosses his results confirmed the larger square.
This led him to his second law, the Law of Independent Assortment, which
states that each pair of alleles segregates independently of each other pair of
alleles during gamete formation. Remember that circle paper activity we did?
That was the Law of Independent Assortment. Each chromosome lined up on the
left or the right and went its merry way without regard to any of the other
chromosomes. So too, went the genes on those chromosomes. Since Mendel’s
day this law has been modified slightly to mean only genes on different
nonhomologous chromosomes, or genes far apart from each other on the same
chromosome. (What, you say? We’ll discuss that possibility next chapter during
gene mapping.) Genes that are close to each other on the same chromosome
tend to be inherited together. There’s a reason. Think about it. Now, get ready
for some math!!!
Rules of probability can be applied to analyze passage of single gene traits from
parent to offspring. In other words, what are the chances that offspring from a
particular mating will have inherited a certain condition? People who have certain
diseases that run in their family may be interested in knowing this kind of thing
before they get married and have children. Genetic counselors can look into a
family’s history and determine, to the best of their ability, what a couples
chances are of having children that have a disorder. Along with some probability
rules we will also be using the Punnett squares to find some numbers, so make
sure you know how to do them.
You have all tossed a coin before, I’m sure. If you toss it once, what are the
chances you will get a head? Correct, one out of two, or ½. Now, if you have
two coins and you toss them both at the same time, if one comes up heads,
what is the other one going to be? What do you mean you don’t know? Why
not? Correct again! What one coin does has NOTHING to do with what the other
coin does. Genes work the same way. Everyone has two genes for each trait, so
at most there are two choices (alleles) as to what is passed to the offspring in a
gamete. Each gene also sorts independently into each gamete and has nothing
to do with other genes on other chromosomes. Sound familiar? It should- those
are Mendel’s two laws.
Now let’s look at the multiplication and addition rules for determining
probability. The multiplication rule states that to determine the probability of two
or more independent events occurring together in some specific combination,
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multiply the probability of one event by the probability of the other event (P(A
and B) = P(A) X P(B)). Look at the monohybrid cross, Rr X Rr, in Fig. 14.9 pg.
270, slide 68. What is the probability of getting rr? It is the probability of getting
r in the sperm and r in the egg that come together in fertilization. That would be
the probability of getting r in sperm (½) times the probability of getting r in the
egg (½) which equals ¼. The same goes for the probability of getting RR.
But what about the heterozygous condition? Rr or rR give the same result
genetically even though the R comes from the egg in one instance and from the
sperm in the other. To find the probability of being heterozygous we need to use
the addition rule, the probability that any one of two mutually exclusive events
will occur is calculated by adding their individual probabilities (P(A or B) = P(A)
+ P(B)). In our example, the probability of Rr = ¼ and rR = ¼ so the
probability of being heterozygous is ¼ + ¼ = ½. Here is a hint to help you
remember which rule to use. If the question asks, what is the probability of
getting this AND that, you multiply; if it ask this OR that then you add.
Sometimes you may have to think a little and possibly reword the question to
figure out if it is and or or, but that’s the key to these problems.
Obviously, you can see from the next two slides in the PowerPoint, that this can
get much more complicated. Go over the example carefully. Actually work out
the individual Punnett squares (not a triple) and do the math so you see where it
is all coming from. We will be practicing this in class too, but I will tell you now
that this will be on the math portion of your test for this unit. You have been
warned.
Inheritance patterns are often more complex than predicted by simple Mendelian
genetics. Characters often times have more than two alleles, they can be
controlled by more than one gene, or the alleles may not be completely
dominant or recessive. Everything we have done so far has been about
complete dominance; one allele in a hybrid is seen and the other is not, so
that the homozygous dominant and the heterozygous individuals look alike.
Define the other two:
Incomplete dominance –
Codominance –
The difference between these two is in incomplete dominance there is a
blending of two traits, like red + white = pink, but codominance would show
both traits at the same time. Instead of pink you would see maybe red spots and
white spots.
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One more thing about dominance: dominant DOES NOT mean more common.
Some recessive genes occur in a population more frequently than dominant
ones. It also does not mean better. Natural selection will choose the right allele
for the given environmental conditions, and over time, that choice may change.
Multiple alleles is a very simple concept. A character would have more than
two alleles available within the population. An individual can never have more
than two alleles, but there can be many spread among many individuals.
Anything more than two is considered many. Scientists only count to three,
remember? Characters that have multiple traits would have multiple alleles. The
ABO blood groups in humans are one example that you should be familiar with.
SOMEBODY ask me in class to go over this and do some problems so that it is
clear to you. The ABO blood groups in humans show multiple alleles,
codominance between A and B, and complete dominance of A and B over i.
A quantitative character is one that varies along a continuum, like going from
short to tall. What does this usually indicate? Explain it and give another example.
Your book also describes two other concepts: pleiotropy and epistasis. These are
NOT covered on the AP exam or your unit test. If you are interested we can
discuss it, otherwise we will skip those.
You do not live in a bubble. Therefore, you interact with the environment and the
environment interacts with you. It also has an effect on your genes. Your book
gives the example of hydrangea flowers. They are nature’s litmus paper. If they
grow in acidic soil the flowers are red, in alkaline (basic) soil they are blue. The
type of soil affects how some genes are expressed. That is true for people as well.
If you do not get enough food you may not reach your proper height or weight; in
summer you will tan when exposed to the sun. Other animals change fur color
with the seasons, and reptiles will develop into males or females depending on the
temperature at which the egg incubates.
There are a lot of factors, both genetic and environmental, that go into making
you who you are. Some genes follow Mendel’s laws and some don’t. It can get
very complicated. However, many traits do and this allows geneticists to be able to
use certain tools to make predictions and find patterns. This helps both in
diagnosing and curing diseases, as well as determining the possible outcome of a
pregnancy.
Even though we don’t make good test subjects, we have memories and keep good
records of past events. These can be used to make pedigrees or family trees. A
pedigree can then be used to determine how a particular genetic disorder is
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passed on, or if there is a chance that someone’s offspring may be afflicted. All
pedigrees are made the same: males are squares, females are circles, and if the
shape is shaded in then that person has the disorder. Horizontal lines connect
spouses and siblings, with vertical lines going to offspring. They can be used to
figure out genotypes and dominant/recessive traits. We’ll do one in class. It’s fun.
Many genetic disorders are recessive, meaning you need two identical alleles to
have that disorder. You would have to inherit the recessive allele from both of
your parents. (Keep this in mind when working on pedigree charts!)
Can a child have a recessive disorder but not either parent?
How? What is the parents’ genotype?
What do we call someone like that?
Why do most cultures, including ours, have laws against marrying close relatives?
What is the most common lethal genetic disease in the United States?
CHAPTER 15 SECTIONS 1-5
If you were to watch mitosis and meiosis take place under a microscope, you
would see that the movement of the chromosomes accounts for Mendel’s laws. We
saw this in our circle activity as well. The chromosomes line up any way they want
(independent assortment), crossing over occurs while they are paired, and then
the homologs separate (segregation). All these things contribute to genetic
variation. At the time when this was figured out, scientists could see the
chromosomes and, due to their behavior, reasoned that Mendel’s “factors”, the
genes, had to be on the chromosomes. Nowadays it is relatively easy to locate a
particular gene, as you can see in slide 98, Fig. 15.1 pg. 286, but back then, no. It
wasn’t until the work of Thomas Hunt Morgan, working with fruit flies, that
anyone was able to actually locate a particular gene.
Fruit flies, Drosophila melanogaster, are small and breed quickly, so you can keep
a lot in a small space. This makes them very convenient for genetic experiments.
It also helps that they have only four pairs of chromosomes. Many people work
with fruit flies still today, and talk about them and experiments are common.
There is a particular way of talking about and coding fly traits. Here’s how it
works. When Morgan worked with the flies he noticed some traits about the flies
that are considered normal. These conditions are called wild type; I guess because
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that is what occurs most often in nature. Any other trait for that character would
be a mutant. When working on problems or Punnett squares, the letters used to
represent the traits come from the mutants and wild type is always noted with a
+. For example, fruit flies have a variety of eye colors, but the wild type is red.
Morgan experimented with white eyed flies. These flies were labeled w and the
wild type red ones were w+. This experiment is shown in slide 106, Fig. 15.4, pg.
289. We won’t be doing genetic experiments with fruit flies, but when you do
some reading about them this convention of labeling traits is standard procedure.
From this experiment, Morgan was able to show that the eye color gene was
located on the X chromosome because the only flies that had white eyes were
male. How does this make sense?
From this experiment we see something else as well. The only flies that had white
eyes were males. That is not normal. With other traits females have the trait too.
The fact that it was only males is what led Morgan to the conclusion that the gene
was on the X chromosome. This is what we call sex linkage and we’ll talk about
that in a minute. The other thing you should get out of this experiment is the fact
that not all traits are inherited following Mendel’s laws. This will be more clear as
we look at more experiments. Remember those ratios from the F1 and F2 squares
I said you should remember? Here is where that will help you. Those ratios are
normal, Mendelian inheritance. Any experiment that doesn’t give those ratios
means there is something else going on, probably gene linkage of some kind.
You learned in the last chapter that, in humans, sex is determined by the last pair
of chromosomes, #23. Females are XX and males are XY. Fruit flies also have XX
and XY. This is not always the case. Some organisms have different ways of
determining sex and you can look in your book, but you won’t be tested on it. As
was demonstrated by Morgan’s experiment, there are other genes on those
chromosomes for other characters that have nothing to do with sex. These genes
are known as sex-linked genes. Sex-linked in humans now refers to a gene
found on the X chromosome. Any time you have genes found on the same
chromosome they are called linked genes.
There are many recessive sex-linked disorders such as hemophilia, color blindness,
and Duchenne muscular dystrophy. These are all much more common in males
than females due to the fact that males only have one X, there is no back-up copy
like females have. The only way females will have the disorder is if both of their
copies of the gene are defective. There is also something in females called X
inactivation that produces Barr bodies. It is explained in your book using calico
cats as an example. It is very interesting and you should read it, but you won’t be
tested on it. If you ever wondered why calico cats are always female, now is your
chance to learn.
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Linked genes do not follow Mendel’s Law of Independent Assortment. They can’t
because they are traveling together on the same chromosome, so when the
chromosomes separate these genes will still be together. Morgan did more
experiments to show this. Let’s look at his experiment with body color and wing
size, Fig. 15.9, pg. 293, slide 117. Morgan did a test cross with an F1 dihybrid
female with b+ b vg+ vg, gray body and normal wings, and a double mutant male
with b b vg vg, black body and vestigial wings. If these genes were on separate
chromosomes you would expect to see a 1:1:1:1 ratio of all possible combinations.
If they were on the same chromosome you would expect to only see the parental
types, black vestigial and gray normal (offspring match one of the parent’s
phenotype), because the genes would stay together. Mostly that’s what he got so
must be those genes are linked. However, look at the actual numbers after results:
965:944:206:185. Where did those last two come from? Those are not parental
types!!!
Those are what we call recombinants, offspring that do not match either
parent’s phenotype but have a new combination of traits. If the genes were on
separate chromosomes and the offspring showed the expected 1:1:1:1 ratio, the
recombination frequency would be 50%, half show parental type and half don’t.
That’s expected. Genes that are linked should not give you recombinants because
those genes are together on one chromosome. But we got some. How did that
happen? What could have happened to mix up those alleles? (Hint: during meiosis)
To move on to the next part, gene mapping, you have to be able to calculate the
recombination frequency. Very easy, don’t worry. Just big words. Here’s how you
do it:
Recombination frequency = number of recombinants X 100
Total offspring
For the example above:
= 206+185 = 391 X 100 = 17%
2300
2300
Morgan had a student named Sturtevant that apparently really got into this stuff
and came up with something called a genetic map or linkage map. What is a
linkage map?
To make a linkage map of a chromosome you will need to be able to calculate the
recombination frequencies between all the genes on the chromosome and then
put them in order. One map unit is equal to one percent of recombination, so the
body color and wing shape genes above are 17 map units apart on the
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UNIT 5 NOTES
chromosome. All this gives you for a map is relative distances of genes NOT
exactly where each one is on the actual chromosome. The farther apart genes are
the more likely it is that they will be separated by crossing over, therefore, the
recombination frequency will be higher and so will their map distance. We will
practice problems in class because you won’t fully understand this until you’ve
done it. It gets mighty tricky when there are several genes involved, but kinda fun.
You’ve now learned all about meiosis, Mendel’s laws, genotypes and phenotypes,
and a lot about chromosomes. This is all great stuff. Yes, it is. And so far all this
has been about the way it is supposed to work. But, do chromosomes always line
up right? Do they always separate correctly? Can something go wrong? Yes,
something can go wrong, and, unfortunately, it is not usually good when it does.
You are designed to function with ONLY two copies of every chromosome, except
for X and Y. This gives you two copies of most of your genes. That’s a good thing;
it gives you a backup copy. So what happens if you only get one and you don’t
have a backup? Or, what if you have three copies? More is always better, right?
Wrong. This is one time where more, or less, is NOT better.
Well, how would you get one more or one less chromosome? They can’t just get
lost, they’re in a cell- there’s no place for them to go except one gamete or the
other. So that’s where they go, but instead of separating normally and going their
separate ways, sometimes chromosomes or chromatids will stay together, they fail
to separate. What is this called?
You can see from Fig. 15.13 pg. 297, slide 131, that this failure can happen at
meiosis I or meiosis II resulting in gametes that have one more (n+1) or one less
(n-1) chromosome. It can be any or even all of the chromosomes. If one of these
gametes happens to be the one that is fertilized, the offspring could be seriously
affected. In the majority of cases spontaneous abortion occurs because normal
development is just not possible. Aneuploidy, refers to any organisms with an
abnormal number of chromosomes. Specifically, monosomy would mean there is
only one copy of a chromosome, and trisomy means there are three. A rare
instance where scientists count to three. Geneticists must be a tad smarter. If an
organism has more than two complete sets of chromosomes it would be called
polyploidy. This condition is much more common in plants than animals, and is, in
fact, how we have gotten some of the cereal grains we eat.
OK, so sometimes the chromosomes stick together. What else can go wrong?
Chromosomes can break. They are just made out of long skinny pieces of DNA
wrapped around proteins. While they are lining up getting ready to separate they
do that crossing over thing. During crossing over they are supposed to exchange
complimentary pieces of DNA. You know, switch genes FrQ for genes fRq. Mix
things up a bit. When this happens each chromosomes has to actually break,
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UNIT 5 NOTES
move, and reattach to the other chromosome. This occurs several times. Again,
this does not always occur flawlessly. There are also times when chromosomes
just break, but either way the result is the same. It can result in any one of these
four types of structural changes:




Deletion removes a chromosomal segment
Duplication repeats a segment
Inversion reverses a segment within a chromosome
Translocation moves a segment from one chromosome to another
For an example of each look at slide 137, Fig. 15.15 pg. 298.
Sometimes these changes in chromosomes will cause a disease or disorder,
sometimes they may go unnoticed. It would depend on which genes were
involved. In humans, for example, there are several disorders that are caused by
chromosomal abnormalities of either number or structure. The most common is
probably Down syndrome, or Trisomy 21, so named for the chromosome of which
there are three. People with this disorder are easily recognized due to the common
facial characteristics associated with the disorder. Nondisjunction can also occur in
the sex chromosomes resulting in Klinefelter syndrome (XXY), Turner syndrome
(X0), or Triple X syndrome (XXX). An X chromosome is necessary for life so there
can not be a Y0 condition.
There are also some disorders caused by structural abnormalities, such as cri du
chat, or chronic myelogenous leukemia (CML). For your AP exam, and maybe the
unit test, you should be able to describe at least one of the following genetic
disorders. You should know its cause, symptoms, etc. Pick one:







Cystic fibrosis pg. 278
Sickle cell anemia pg. 278
Tay-Sachs disease pg. 272
Huntington’s disease pg. 279
X-linked color blindness pg. 290
Trisomy 21/Down syndrome pg. 299
Klinefelter’s syndrome pg. 299
Now that you know almost all there is to know about genes, chromosomes, and
heredity, it’s time to explain the exceptions. Anytime there are rules, there are
always exceptions. Here there are only two, and one you don’t need to know
about. That is genomic imprinting. It’s a funky thing that happens where some of
your alleles will be ‘turned off’ depending on whether you got the gene from your
mom or your dad. This concept is not required learning so we will skip it.
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UNIT 5 NOTES
You DO need to know about the extranuclear genes that you have. And you do
have some. Chloroplasts, mitochondria, and other plant plastids contain small,
circular pieces of DNA that contain functional genes. These organelles reproduce
on their own within the cell and copies of their DNA would go to the daughter
organelles. However, when the cell divides these organelles are randomly put into
one daughter cell or the other. Any traits determined by these genes would not
follow Mendel’s rules since there is no meiotic sorting. Some cells may do one
thing while others do something different, as you can see by the spotted plant in
Fig. 15.19 pg. 301, slide 146.
Sperm contribute only nuclear genes (the 23 chromosomes) to offspring. The egg
has all the necessary organelles. Most of the mitochondrial genes code for proteins
in the ETC and ATP synthase. If any of these genes are defective production of
ATP would suffer and there would be a lack of energy. Muscle and nerve cells are
the most affected. Mitochondrial myopathy is one such mitochondrial disorder.
Mitochondrial DNA can also be used to trace maternal lineage.
You can use this final slide from the PowerPoint to make sure you understand
linkage and independent assortment.
Fig. 15-UN4
Sperm
P generation D
gametes
C
B
A
Egg
E
c
b
a
d
+
F
f
The alleles of unlinked
genes are either on
separate chromosomes
(such as d and e) or so
far apart on the same
chromosome (c and f)
that they assort
independently.
This F1 cell has 2n = 6
chromosomes and is
heterozygous for all six
genes shown (AaBbCcDdEeFf).
Red = maternal; blue = paternal.
D
Each chromosome
has hundreds or
thousands of genes.
Four (A, B, C, F) are
shown on this one.
e
e
C
B
A
d
E
F
c
ba
17
f
Genes on the same chromosome whose alleles are so
close together that they do
not assort independently
(such as a, b, and c) are said
to be linked.
UNIT 5 NOTES
These are some important things you should think about. Some of you will have to
answer questions like these someday. Part of my job is to help you get ready and
be informed to answer these questions. I have assigned you a book to read that
brings up many of these topics and we will have discussions in class. I expect you
to ask questions, offer opinions, and talk openly about this stuff. Some of it may
be difficult. If anything is really uncomfortable, let me know. The idea is to inform
and educate you, not make you upset. Please, also keep in mind that there are no
right or wrong answers here. There are two sides to almost all these issues, and
no one can predict the future. These are tough questions, but some that you may
need to answer someday.
Many ethical, social and medical issues surround human genetic
disorders.
 Reproduction issues
 Civic issues such as ownership of genetic information, privacy, historical
contexts, etc.
Who do you think should own your genetic information? Should your employer,
bank, or insurance company have access to this information? Would you consider
being tested for a genetic disorder? If such testing were commonplace would you
want to know the results? If your unborn child tested positive for a genetic
disorder would you consider abortion? Should doctors be allowed to keep your
‘unwanted’ organs after surgery?
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